UV LED linear curing

A linear light source produces a long, narrow band of UV light. Mounted across a conveyor, a web or a rotating part, it cures the material as it passes underneath. Linear curing is the natural choice whenever the product moves: printing and coating lines, conveyors, long bond lines and cylindrical parts that turn under the lamp.

How it works

The light source is a row of UV LEDs behind a window, typically 3 to 10 mm wide and from about 10 mm to more than a metre long. A controller sets the output and switches the LEDs on and off, either continuously while the line runs or triggered by a sensor as each part arrives.

Every point on the product receives light only while it is under the band. The dose it collects therefore depends on three things: the irradiance under the lamp, the width of the irradiated band and the speed at which the product moves.

Irradiance, dose and line speed

Irradiance (mW/cm²) is the UV power per unit area reaching the surface. Dose (mJ/cm²) is the energy per unit area, built up over the time a point spends under the lamp:

• Dwell time = width of the irradiated band ÷ line speed
• Dose ≈ average irradiance across the band × dwell time

As an illustration, a band 10 mm wide on a line moving at 10 m/min (about 167 mm/s) gives a dwell time of 0.06 s. At an average irradiance of 1,000 mW/cm², that is about 60 mJ/cm² per pass.

Two consequences follow. Doubling the line speed halves the dose. And a higher peak irradiance does not raise the dose if the band becomes narrower. To reach a higher dose at the same speed, use a source with more output, a wider emitting area, several sources in a row, or more passes.

The irradiance across a linear source is not perfectly flat: it peaks in the centre of the band and falls off towards the edges and the ends. Measure the actual dose with a radiometer that travels through the curing zone on the conveyor and records the accumulated energy.

Working distance

Line sources are usually specified at a working distance of about 10 mm. The band widens and the irradiance falls as the distance increases, so mount the source as close to the product as the process allows, at a fixed and repeatable height. On conveyors with adjustable lamp height, re-measure after each change.

Wavelength

Linear sources are available at 365, 385, 395 and 405 nm. Use the wavelength your ink, coating or adhesive is formulated for. 365 nm suits many adhesives and formulations derived from mercury-lamp chemistry; 385 and 395 nm are widely used for LED-curable inks and coatings; 395 and 405 nm penetrate further into thick or pigmented layers.

Typical uses

• Ink and varnish curing on label, inkjet and coding lines in Printing
• Film coating, wood finishes and adhesives on moving lines in Coatings and Adhesives
• Edge and bond-line curing in Electronics and fibre arrays in Optical Fibre Communication
• Curing around cylindrical parts that rotate under the lamp
• Curing flat parts on a belt in a UV Conveyor

Limits

• Short dwell time. The narrow band gives each point only a brief exposure. Slow-curing or thick layers may need a wider source, a lower line speed or several passes. For stationary parts, flood curing is usually a better fit.
• End fall-off. Irradiance drops near the ends of the source, so choose a source somewhat longer than the product width.
• Heat build-up at high power. The LEDs emit no infrared, but high-power sources generate heat that has to be removed by fans or water cooling.
• Surface cure. Without the short UV wavelengths of a mercury lamp, some formulations stay slightly tacky on the surface. LED-optimised chemistry or higher irradiance solves this.

How to specify a linear system

1. Product width, plus a margin, to set the source length.
2. Line speed and the required dose from the material data sheet, which together set the irradiance and band width you need.
3. Wavelength, from the material data sheet.
4. Working distance and the mounting space above the line.
5. Cooling: fan cooling for most applications, water cooling for the highest irradiance and enclosed machines.
6. Control: continuous or triggered operation, PLC and RS232 interfaces, and status and alarm outputs.

Awellcure linear light sources

Awellcure UV LED Linear sources range from compact 10 × 3 mm and 15 × 5 mm heads for narrow bond lines to fan-cooled bars from 50 × 10 mm up to 1,350 × 10 mm for web and conveyor widths. For higher irradiance in a line format, the water-cooled 20 mm wide sources in the UV LED Area range reach 6 to 25 W/cm². Their controllers accept external trigger signals from a PLC, sensor or foot switch, provide status and alarm outputs, and can be programmed through RS232.